Alkyl aluminum sesquihydrides



3,535,261 ALKYL ALUMINUM SESQUIHYDRIDES Paul Kobetz and Roy J. Laran,Baton, Rouge, 1.21., assignors to Ethyl Corporation, New York, N.Y., acorporation of Virginia Original application Dec. 17, 1963, Ser. No.331,202, now Patent No. 3,453,093, dated July 1, 1969. Divided and thisapplication Jan. 27, 1969, Ser. No. 816,470

Int. Cl. C07f 5/06 US. Cl. 252-182 3 Claims This is a division ofapplication Ser. No. 331,202, Dec. 17, 1963, now Pat. No. 3,453,093,July 1, 1969.

This invention relates to the manufacture of complex bimetallichydrides, and in particular, sodium aluminum hydride, NaAlH Moreparticularly, the invention relates to a new and improved processwhereby this complex metal hydride is readily made at low pressures anda concurrent product is a dialkyl aluminumm hydride.

Sodium aluminum hydride is a known commodity and has heretofore beenmade by the reaction of aluminum trichloride and sodium hydride. It isuseful for the reduction of organic compounds and for generation ofhydrogen gas by controlled reaction with water.

Dialkyl aluminum hydride, a concurrent product of the present process,is a known compound. Diethyl aluminum hydride has been made by thereaction of triethyl aluminum, aluminum, and hydrogen, as in US. Pat.2,787,626. In the case of dimethyl aluminum hydride, this product hasnot been commercially available, although its preparation by thereaction of sodium hydride and dimethyl aluminum chloride has beenknown. The dialkyl aluminum hydrides are useful as catalysts or catalystcomponents for the polymerization of lower olefins.

The object of the present invention is to provide a new and convenientprocess for the manufacture of sodium aluminum hydride, NaAlH Anotherobject is to provide a process for the concurrent production of adialkyl aluminum hydride, the alkyl groups of this product having fromone to two carbon atoms, viz, including the methyl and ethyl groups. Anadditional object is to provide certain inert compositions, namely,solutions of alkyl aluminum sesquihydrides.

Details of the invention and of the best mode of its conduct will beclear from the detailed description and examples given hereinafter andfrom the accompanying figure which is a schematic representation of theprocess sequence operations in a particularly preferred embodiment ofthe invention.

It has been discovered that sodium aluminum hydride and a dialkylaluminum hydride are readily developed by the reaction of sodium hydrideand an alkyl aluminum dichloride. The stated reactants can be reacted intwo discrete stages, but in every instance the overall reaction whereinR represents an alkyl group, consisting of carbon and hydrogen, andhaving from one to two carbon atoms.

The alkyl aluminum dichloride which is an essential reactant for theprocess may be provided as is, viz, unaccompanied by any othercomponent, or may be in the presence of a dialkyl aluminum chloride, themixture being represented by the common terminology of an alkyl aluminumsesquichloride, in which instance the overall reaction is It is seenfrom Equations 1 and 2, that, when an alkyl aluminum sesquichloride isused as the initial feed material, the relative proportion of dialkylaluminum hydride produced concurrently with the sodium aluminum hydride3,535,261 Patented Oct. 20, 1970 is tripled. Alternatively, then, whenthe sodium aluminum hydride production is to be maximized, it ispreferable to employ as the initial aluminum containing feed materialthe alkyl aluminum dichloride, viz, methyl aluminum dichloride or ethylaluminum dichloride.

It will be further noted that sodium chloride is concurrently produced,and as is shown hereinafter, this necessitates a separation step toobtain relatively pure sodium aluminum hydride.

In a preferred category of embodiments, a two-stage process is employedwherein the alkyl aluminum chloride component is reacted with only partof the sodium hydride initially in a hydrocarbon reaction medium, andall the chloride content is converted to sodium chloride which isprecipitated and separated. The resultant solution, then containing onlyalkyl groups and hydrogen bonded to the aluminum content, is reactedfurther with additional sodium hydride, which results in the formationof sodium aluminum hydride and dialkyl aluminum hydride.

Equations representing, generally, a two-stage operation as outlinedabove, are, for the case of using an alkyl aluminum sesquichloride as afeed material, as follows:

In an operation as illustrated by Equations 3 and 4 above, the firstreaction is conducted in an inert liquid hydrocarbon reaction mediumwherein the sodium chloride formed by the reaction of the sodium hydrideand the chlorine content is precipitated. The material in solution isthen termed an alkyl aluminum sesquihydride, R Al H and is considered asan equimolal mixture of dialkyl aluminum hydride and alkyl aluminumdihydride, the latter component being stabilized by the dialkylcompound. The liquid phase remaining after the sodium chloride isseparated in the first stage is then reacted with additional sodiumhydride according to Equation 4, which forms sodium aluminumtetrahydride and a dialkyl aluminum hydride. This second reaction isnormally carried out in the same solvent remaining from the first stage.The sodium aluminum hydride does not precipitate, however, apparentlybecause of complexation in soluble form of the dialkyl aluminum hydrideand the sodium aluminum hydride. To release solid sodium aluminumhydride, a lower alkyl dialkyl ether can be added. Thus, diethyl etherresults in precipitation of the sodium aluminum hydride and allows itsseparation by filtration and drying, leaving the dialkyl aluminumhydride as a loose complex with the ether dissolved in the solvent.

In the embodiments of the process wherein the feed material is an alkylaluminum dichloride, a highly effective process involves the recycle ofa portion of the dialkyl aluminum hydride formed as a joint product, asis illustrated schematically by reference to the figure.

Referring to the figure, in a first stage reaction, an alkyl aluminumdichloride and sodium hydride are reacted in the presence of arecirculated quantity of dialkyl aluminum hydride in approximatelyequimolar proportions to the alkyl aluminum dichloride, the sodiumhydride being provided in proportions of about two moles per mole ofalkyl aluminum dichloride. The reaction is carried out in an inerthydrocarbon solvent such as benzene, toluene, or in other aromatic oraliphatic liquid hydrocarbons. As a result of the reaction therein, asindicated by Equation 5 below, a solution of an alkyl aluminumsesquihydride is produced. The function of the recirculated dialkylaluminum hydride is, apparently, to stabilize the alkyl aluminumdihydride which is the result of the reaction of the alkyl aluminumdichloride and the sodium hydride, as indicated by Equation 6.

The next step is the separation of sodium chloride produced as aninsoluble precipitate, by filtration, centrifuging or some otherliquid-solid separation technique. The liquid phase remaining is thensubjected to reaction with an additonal mole of sodium hydride. A liquidphase is produced, and in order to isolate solid crystalline sodiumaluminum hydride, diethyl ether or a similar lower dialkyl ether inwhich sodium aluminum hydride is insoluble is added thereto and resultsin precipitation of the sodium aluminum hydride. The thus formedprecipitate is separated and subsequently purified further, if desired,and the resultant liquid phase is stripped of the first solvent, atleast in part, by a vacuum distillation or similar partial pressureseparation process. The dialkyl aluminum hydride forms a complex with alower-dialkyl ether, but this is readily resolvable by heating wherebythe diethyl ether or simiar dialkyl ether is recovered for re-use, andthe dialkyl aluminum hydride product is obtained in good yield. Thedialkyl aluminum hydride is withdrawn as a product only to the extent ofabout one-third of the eflluent stream at this point.

Working examples of the process and the several variations thereof aregiven below:

EXAMPLE 1 A supply of methyl aluminum sesquichloride in benzene solutionwas added to a slurry of sodium hydride in benzene. The methyl aluminumsesquichloride solution was added in a dropwise manner while stirringthe mixture vigorously, the addition being over a period of threehour-s. The sodium hydride slurry contained about 24 weight percentsodium hydride based on the benzene, and was used in proportions ofalmost exactly three moles per mole of methyl aluminum sesquichloride.The methyl aluminum sesquichloride was in solution in benzene inconcentration of about 73 weight percent. The reaction was conducted atroom temperature and sodium chloride was steadily precipitated duringthe entire reaction period of three hours.

The slurry from the above reaction was filtered, and the cake washedwith benzene, the washings being added to the filtrate.

In the second step of the operation, the clear filtrate solution hadsodium hydride added thereto, in the proportions of about one mole per4.6 atoms of the aluminum content of the solution. In other words, theproportion of sodium hydride reaction in the second stage was in theproportion of about one mole per 2.3 moles of methyl aluminumsesquihydride in the solution. The reaction was conducted at around 60C. and no precipitation occurred. Cooling of the reacted mixture to 10C. resulted in some small precipitation, but addition of diethyl ether,in the proportions of about 40 volume percent of the solution, resultedin prompt saltin-g out of white solids. These were filtered, washed withether, and dried, and a yield of about 28 parts of dried material wasobtained. Analysis by hydrolysis and measurement of hydrogen evolvedshowed that the sodium aluminum hydride product had a purity ofapproximately 91 percent, and the overall yield, based upon the methylaluminum sesquichloride initially fed, was over 80 percent of puresodium aluminum hydride content.

The filtrate remaining after removal of the sodium aluminum hydridesolids is a solution of a complex of diethyl ether with dimethylaluminum hydride. If desired to recover the dimethyl aluminum hydride assuch, the benzene and ether are removed by distillation under a partialvacuum. A high yield of dimethyl aluminum hydride, a colorless volatileliquid, is obtained.

4 EXAMPLE 2 As previously indicated, the process and the severalvariations thereof are generally applicable to processing both methyland ethyl compounds. Thus, when the operations of the preceding exampleare repeated, but instead of a feed consisting of methyl aluminumsesquichloride, ethyl aluminum sesquichloride, (C H Al Cl is substitutedfor the methyl aluminum sesquichloride, comparable efiiciency isobtained, and the joint product with the sodium aluminum hydride is ahigh purity diethyl aluminum hydride.

EXAMPLE 3 As previously indicated, the starting material, instead ofbeing an alkyl aluminum sesquichloride mixture, can be an alkyl aluminumdichloride. Such compounds are readily made by the interreaction of asesquichloride compound and aluminum chloride in the appropriateproportions according to the general equation In a typical operationusing such a material, one mole of methyl aluminum dichloride is mixedwith a mole of recirculated dimethyl aluminum hydride, in a benzenesolution, and reacted by techniques similar to that already describedwith a suspension of sodium hydride in an inert alkane hydrocarbon suchas 2,2,3-trimethyl pentane. The sodium hydride is added in approximatelythe proportions corresponding to the atoms of chlorine provided in themethyl aluminum dichloride fed. The reaction proceeds smoothly,resulting in a solution of methyl aluminum sesquihydride, (CH Al H inthe hydrocarbon, and precipitated sodium chloride. The sodium chlorideis removed by a careful filtration, and the hydrocarbon solution is thenreacted with additional sodium hydride, in the proportions of about onemole per four atoms of the aluminum content of the solution, andthereafter a low alkyldialkyl ether, such as dimethyl ether, diethylether, methyl ethyl ether, diisopropyl ether, or the like, is added tothe liquid phase, and a sodium aluminum hydride product is crystallizedout. This is readily separated by filtration, and provides, afterdrying, a high purity good quality product. About two-thirds of thedimethyl aluminum hydride remaining in solution is recirculated to thefirst reaction stage.

The function of the recirculated dialkyl aluminum hydride is notprecisely understood, but is believed to provide a stabilizing action byimmediate complexing with the theoretical compounds methyl aluminumdihydride which results upon the reaction of methyl aluminum dichlorideWith the appropriate quantity of sodium hydride. This stabilizing actionis highly desirable when a high purity sodium aluminum hydride isdesired, viz., unaccompanied by sodium chloride solids, or when asolution of an alkyl aluminum sesquihydride is desired as a sideproduct.

EXAMPLE 4 When ethyl aluminum dichloride is substituted as the feedmaterial in the first reaction stage of Example 3, comparable reactionis obtained, and the products are sodium aluminum hydride and diethylhydride.

In those cases wherein the presence of sodium chloride in the sodiumaluminum hydride product is not objectionable, its removal after thefirst reaction stage is not necessary, and the several reactions can bemerged into one. Under such circumstances, the alkyl aluminum dichlorideis reacted in a single stage with sodium hydride in the proportions ofabout five moles or thereabouts of sodium hydride to two moles of thealkyl aluminum dichloride. The products are a precipitate of a mixtureof sodium chloride and sodium aluminum hydride in the proportions ofabout 20 mole percent sodium aluminum hydride concentration. Thisco-precipitate can be resolved and a good quality sodium aluminumhydride removed, by extraction of the filtered and dried solids with asolvent for the sodium aluminum tetrahydride, viz., tetrahydrofuran orthe lower dialkyl ethers of lower polyalkylene glycols.

From the foregoing description and examples, it will be seen that theprecise techniques of operation can be appreciably varied. Generally,the specific identiy of the solvent employed for the reaction medium isnot highly critical, but of course the solvent should be non-reactivewith the reactants and should not be a solvent for sodium chloride.Aromatic hydrocarbons as specifically illustrated, and normally liquidalkane hydrocarbons or mixtures of hydrocarbons are highly effective.Thus, 'when hexane, pentane, naphthas, or similar hydrocarbon compoundsor mixtures are substituted in the foregoing examples for the solventsillustrated for the first reaction, comparable results will be obtained.The sodium hydride, being a solid, is desirably pre-slurried in thereaction medium.

As, in all cases, a joint product is the corresponding dialkyl aluminumhydride, for full resolution of the two products, precipitation of thesodium aluminum hydride is essential and this is accomplished, asindicated, by the addition of an appropriate quantity of a lower dialkylether.

Temperatures of operation are not critical and can vary from, normally,about 0 C. to about 150 C., but the preferred temperatures are fromabout 20 to 60 C. Similarly, the pressure of operation is normallyambient pressure, or the vapor pressure of the reacting mixture at thetemperature of operation.

The precise proportions of sodium hydride used are not highly critical.Thus, variations of plus or minus ten percent of the theoreticalrequirements for the several reactions are permissive. For best results,variations of not more than two percent are desirable.

The methyl aluminum sesquihydrides, or ethyl aluminum sesquihydride,solution produced as an intermediate by the reaction of sodium hydridewith the corresponding alkyl aluminum is useful in its own right. It issusceptible to conversion, by olefin addition, to mixed alkyl trialkylaluminum compounds, or as a component of polymerization catalystsystems.

Having described the process, what is claimed is:

1. A composition consisting essentially of an inert liquid hydrocarbonsolution of an alkyl aluminum sesquihydride, the alkyl groups havingfrom one to two carbon atoms.

2. A composition consisting essentially of an inert liquid hydrocarbonsolution of methyl aluminum sesquihydride.

3. A composition consisting essentially of an inert liquid hydrocarbonsolution of ethyl aluminum sesquihydride.

References Cited UNITED STATES PATENTS 2,915,541 12/1959 Ziegler et al.260448 2,915,542 12/1959 Robinson et al. 260448 MAYER WEINBLATT, PrimaryExaminer I. GLUCK, Assistant Examiner US. Cl. X.R.

1. A COMPOSITION CONSISTING ESSENTIALLY OF AN INERT LIQUID HYDROCARBONSOLUTION OF AN ALKYL ALUMINUM SESQUIHYDRIDE, THE ALKYL GROUPS HAVINGFROM ONE TO TWO CARBON ATOMS.